This invention relates to a method for measuring the concentration of the NOx gas and an apparatus or sensor for carrying out the measurement method. More particularly, it relates to a method for measuring the NOx gas concentration used in exhaust gases containing a large quantity of H2O, and a NOx gas concentration sensor.
Recently, researches into directly measuring the concentration of the NOx gas for controlling the infernal combustion engine or catalysts are proceeding in meeting with intensification of regulations on vehicle exhaust gases.
In particular, researches are going on extensively for a NOx gas concentration sensor of the type in which the oxygen is pumped out by a first oxygen ion pump cell using a solid electrolyte (oxygen ion conductor) such as zirconia, so as not to cause decomposition of the NOx gas (2NOxe2x86x92N2+O2) by the first oxygen ion pump cell and in which oxygen is further pumped out from the NOx-gas-containing residual gas by the second oxygen ion pump cell for decomposing NOx and measuring the current generated by dissociated oxygen ions for detecting the NOx gas concentration because measurement of the NOx gas concentration is thought to be easily made without being affected by interfering gases contained in the exhaust gases, such as HC or CO.
In the course of investigation toward the present invention following problem has been encountered.
According to the finding reached by the present inventors, the above method has a drawback that, if H2O is mixed into the NOx gas concentration sensor, the NOx concentration cannot be measured accurately.
It is therefore an object of the present invention to provide a method for measuring the NOx gas concentration for enabling accurate measurement of the NOx gas concentration in a H2O containing atmosphere and a NOx gas concentration sensor for carrying out the method.
Other problems and objects of the present invention will become apparent in the entire disclosure.
Among various aspects of the present invention there are the following elements. The elements included in aspect (1-1) are as follows: there is a first step of introducing the gas under measurement containing NOx into the flow channel. The oxygen gas is drawn or evacuated to outside of the flow channel via a ceramic body having an electrically controllable conducting rate of oxygen. A NOx concentrated residual gas is formed in the flow channel. A voltage is applied to the ceramic body to an extent that does not substantially dissociate H2O in the residual gas. Particularly, a voltage lower than the dissociation voltage between hydrogen and oxygen of at least H2O (Hxe2x80x94O dissociation voltage) in the residual gas is impressed across the electrodes formed on the ceramic body. NOx in the NOx concentrated residual gas is dissociated into nitrogen and oxygen. The current caused by NOx dissociation flowing in the ceramic body is measured across the electrodes. The current is generated by the electro-chemical action of oxygen dissociated from the NOx gas during the dissociation step. The NOx concentration is determined based on this current.
According to further aspect based on aspect (1-1), the flow of the gas into the flow channel is restricted (aspect 1-2). The flow of the residual gas to the electrode for the NOx dissociation is restricted (aspect 1-3).
The flow channel comprises a first flow channel and a second flow channel communicating with the first flow channel, the formation of the residual gas for concentrating Nox without dissociating H2O is performed in the first flow channel, and NOx is dissociated in the second flow channel (aspect 1-4).
According to aspect (2-1), there is provided a NOx gas concentration sensor for measuring the NOx concentration on a gas under measurement. The sensor comprises a ceramic body having an electrically controllable conducting rate of oxygen ion; a flow channel provided facing said ceramic body so that the NOx containing gas under measurement will flow therein, with the NOx gas in the gas under measurement being varied in the concentration during the process of movement of the gas under measurement therethrough; and means (generally pumping cell) for extracting the oxygen gas from the gas under measurement via said ceramic body to outside of the flow channel for forming a residual gas, in said flow channel, having a NOx concentration different from before entering the flow channel; and a plurality of electrodes formed on said ceramic body so that NOx in the residual gas is dissociated into nitrogen and oxygen by applying a voltage to an extent that H2O in the residual gas is substantially not dissociated. The sensor further comprises generally circuit means, i.e., means for measuring a current flowing in said ceramic body across said electrodes, said current being generated by the electrochemical operation of oxygen dissociated from the NOx gas; and means for determining the NOx concentration in the gas under measurement based on said measured current.
According to aspect (2-2), in the flow channel there is disposed an electrode for extracting oxygen gas out of the flow channel along the flowing direction of the measurement gas, and
wherein the electrode has a length relative to the entire length of the flow channel being:
electrode/entire length=xc2xcto xc2xe.
According to aspect (2-3), the flow channel comprises a first flow channel in which said residual gas is formed, and a second flow channel in which NOx is dissociated, the second flow channel having an inlet communicating to the first flow channel, and the electrode disposed in the flow channel for extracting the oxygen gas out of the flow channel extends from the vicinity of a gas flow restrictor disposed in the first flow channel until at longest a position closer to said gas flow restrictor than the inlet of the second flow channel.
In the NOx gas concentration sensor, the sensor output IP2) usually has temperature dependency and oxygen concentration dependency, thus obstructing accurate measurement of the NOx gas concentration. The NOx gas concentration sensor according to the aspects 2-2 or 2-3 exhibits low oxygen concentration dependency to enable accurate measurement of the NOx gas concentration. If the sensor according to the aspects 2-2 or 2-3 is mounted on the exhaust system of an internal combustion engine driven in the vicinity of a lean range or a theoretical value point (stoichiometrical point), the NOx gas concentration detection output is stabilized in case of lean-rich switching, that is in case of acute change in the oxygen concentration in the exhaust gases.
According to further aspect (2-4), the NOx gas concentration sensor is formulated as follows. The ceramic body is formed of plural ceramic layers laminated together; the flow channel includes a first flow channel for forming residual gas and a second flow channel for dissociating NOx, the first and second flow channels extending along the surface of said laminated ceramic layers; and the first and second flow channels communicate with each other by a flow channel extending in the laminating direction of the ceramic layers.
According to aspect (2-5), in the NOx gas concentration sensor1, an oxygen concentration detecting electrode for detecting the oxygen concentration in the flow channel is formed on the ceramic body facing the flow channel, wherein an oxygen concentration for generating a reference potential versus the oxygen concentration detecting electrode is formed on the ceramic body outside of the flow channel; the oxygen concentration reference electrode is an auto-generating reference electrode across which a voltage is impressed for transporting oxygen towards the reference electrode for providing a constant oxygen concentration around this reference electrode.
According to aspect (2-6), the electrode provided outside the channel for drawing the oxygen gas to outside of the flow channel is sealed or covered. This electrode communicates with atmosphere via a lead and is provided with a gas diffusion resistance.
According to aspect (2-7) the electrode provided in the flow channel for drawing the oxygen gas to outside of said channel, a solid electrolyte component having oxygen ion conductivity carries a component inhibiting the NOx dissociation.
Further aspects relating to the structure of the sensor will be later disclosed in more detail with reference to the embodiments of the present invention.
Further aspects relating to the method for measuring the Nox gas concentration generally relate to aspects (3-1) to (3-4), which correspond to claims 12 to 15, respectively.
According to aspect (4-1), there is provided a NOx gas concentration sensor comprising :a first measurement chamber into which a measurement gas is introduced via a first diffusion resistance; an oxygen partial pressure detection electrode for measuring the oxygen partial pressure in the measurement gas in the first measurement chamber; a first oxygen ion pump cell for pumping out the oxygen in the measurement gas in the first measurement chamber therefrom, based on a potential of the oxygen partial pressure detection electrode, to a sufficient extent such as not to decompose NOx in the measurement gas; a second measurement chamber into which a gas is introduced from the first measurement chamber via a second diffusion resistance; and a second oxygen ion pump cell having paired electrodes on inner and outer sides of the second measurement chamber, the second oxygen ion pump cell being flown through by a current caused by oxygen dissociated on decomposition of NOx in the second measurement chamber on impression of a voltage across the paired electrodes. In this sensor, the voltage impressed across the paired electrodes of the second oxygen ion pump cell is set such as substantially not to dissociate H2O present in the second measurement chamber.
According to aspect 3-1, the voltage impressed across the paired electrodes disposed on inside and outside of the measuring chamber forming a second oxygen ion pump cell is set such as substantially not to dissociate H2O present in the measurement chamber. This measurement method is applied to NOx gas concentration detector having a first measurement chamber into which a measurement gas is introduced via a first diffusion resistance, an oxygen partial pressure detection electrode for measuring the oxygen partial pressure in the measurement gas in first measurement chamber, a first oxygen ion pump cell for pumping out the oxygen in the measurement gas in the first measurement chamber to outside of the chamber, based on a potential of the oxygen partial pressure detection electrode, to a sufficient extent of not decomposing the NOx in the measurement gas, a second measurement chamber into which a gas is introduced from the first measurement chamber via a second diffusion resistance and a second oxygen ion pump cell having paired electrodes on an inner and outer sides of the second measurement chamber, the second oxygen ion pump cell being flown through by the current generated by oxygen dissociated on decomposition of NOx in the second measurement chamber on impression of a voltage across the paired electrodes, in which the current flowing through the second oxygen ion pump cell is measured for finding the NOx gas concentration from the current. By setting the voltage impressed across paired electrodes provided on the inner and outer sides of the second oxygen ion pump cell to a pre-set value, it becomes possible to prevent dissociation of H2O in the second measurement chamber.
With the aspect 3-1, since dissociation and decomposition of H2O is substantially prevented from occurring in the second measurement chamber, the pump current in the second oxygen ion pump cell is not increased due to oxygen generated by dissociation of H2O thus maintaining NOx gas concentration calculation (convention) accuracy (effect (1)). In particular, by controlling the oxygen concentration in the second measurement chamber to a condition precluding H2O dissociation, it becomes possible to improve the H2O concentration dependency of the offset value of the second oxygen ion pump cell current Ip2.
In aspect 3-2 or 3-3 of the present invention, based on the above-mentioned aspect 3-1, since the voltage applied across the second oxygen ion pump cell can be set to 300 to 400 mV or to 400 to 500 mV if the electrode provided on the outer side of the second oxygen ion pump cell is or is not exposed to the atmosphere under measurement, respectively, not only can the effect (1) displayed, but also there occurs (2) decomposition of NOx gas (primarily NO) sufficiently to increase the pump current of the second oxygen ion pump cell produced on NOx gas decomposition, that is to increase the gain, such as to maintain NOx gas concentration measurement (convention or calculation) accuracy.
Preferably, the voltage is set to the former or the latter if the electrodes are provided on both sides of the solid electrolyte layer and the electrode on the outer side of the flow channel is substantially exposed, or if both electrodes are provided on one side of the solid electrolyte layer (one of the electrodes being preferably isolated from the atmosphere in the flow channel), respectively.
In aspect 3-4, which is desirable in connection with the above-mentioned aspect 3-1, oxygen in the gas under measurement is pumped out by the first ion pump cell so that the voltage of the oxygen partial pressure sensor electrode will be equal to 150 to 450 mV. This enables the oxygen concentration in the gas diffused into the second measurement chamber to be lowered sufficiently such as not to substantially decompose NOx thus allowing for correct measurement of the NOx gas concentration in an atmosphere which is only slightly affected by the residual oxygen.
The present inventors have completed the present invention based on the following information. First, the NOx gas concentration sensor by the NOx gas concentration sensor which preferably carries out the measurement method of the present invention, according to the JP Patent application 8-160812, filed by the present applicant (Assignee), is explained. There is proposed, in this earlier application, a sensor having two sets of solid-electrolyte oxygen ion pump cells, two sets of measurement chambers communicating with each other via a diffusion port extending along the stacked direction of the cells and an oxygen partial pressure sensor electrode for measuring the oxygen partial pressure in the first measurement chamber into which is introduced the gas under measurement via a diffusion port. With the sensor, oxygen is pumped out from the first measurement chamber by the first oxygen ion pump cell so that NOx is not decomposed in the first measurement chamber and so that the residual oxygen concentration will be lowered, whereas NOx is decomposed by the second oxygen ion pump cell and the current flowing in the second oxygen ion pump cell is measured for detecting the NOx gas concentration. With this sensor, if the electrode provided on the outer side of the second oxygen ion pump cell is exposed to the atmosphere under measurement, the first oxygen ion pump cell is controlled so that the potential of the oxygen partial pressure sensor electrode is 150 to 350 mV, while the voltage impressed across the second oxygen ion pump cell is set to 450 mV. It has turned out that, if the voltage impressed across the second oxygen ion pump cell is set in this manner, the NOx gas concentration cannot be measured accurately in case H2O is mixed into the NOx gas concentration sensor.
The present inventors have conducted further investigations and have found the followings: (1) If H2O comes into the NOx gas concentration senor, the pump current flowing in the second oxygen ion pump cell is increased to render it impossible to make accurate measurement of the NOx concentration; (2) the amount of oxygen flowing in the second oxygen ion pump cell is changed with the oxygen concentration in the second measurement chamber thus causing fluctuations in the offset; and (3) NOx decomposition characteristics are changed depending on the oxygen concentration in the atmosphere thus causing fluctuations in the gain. The offset is the value of Ip2 in case NO is not injected into the gas under measurement. It is desirable for the offset to be smaller in value. Moreover, it is desirable for the offset to be less sensitive to fluctuations in the extraneous influencing factors, such as the oxygen concentration or temperature, such that it can hardly be changed due to these fluctuations. In the present invention, the xe2x80x98gainxe2x80x99 means the difference in the second oxygen ion pump cell current Ip2 at a NO gas concentration of 0 ppm and that at a pre-set concentration (1000 ppm).
The present inventors also have made the following findings: (4) If the limiting current characteristics of the second oxygen ion pump cell are measured, the second ion pump cell voltage specifying the limit current area is significantly changed due to the presence of H2O; (5) in this limit current area, there is a second ion pump cell voltage area in which the atmosphere oxygen concentration dependency is hardly observed; (6) although the H2O dissociation voltage is thought to be of the order of a few volts, there is probability that the H2O dissociation voltage is lowered under a low oxygen concentration. That is, the dissociation voltage of NOx is approximately equal to that of H2O as an obstructive gas, so that there is a probability that the moisture plays a significant role in measurement of the NOx gas concentration.
In one aspect for the formation of the residual gas either in the flow channel or in the first flow channel, the voltage is applied preferably to an extent that does not substantially dissociate H2O and/or CO2.
In a still further aspect, the residual gas is formed under the condition that allows and compensates for substantial decomposition of NO in the first flow channel or generally an upstream region in the flow channel. This condition is established such that the in-flowing NOx from the ambient gas compensates the decomposed amount of NO to bring an equilibrium state in the residual gas.
The NOx is understood to be substantially the sum of NO and an amount of NO2 in an equilibrium determined generally by the temperature in which the rate of NO2 decreases as the temperature rises. For example, at room temperature it is almost NO2, at 300-400xc2x0 C. 50/50 and at 700xc2x0 C. or above NO2 is 5% or less.
The sensor is preferably operated at about 700xc2x0 C. or above and up to about 900xc2x0 C. more preferably at 750-850xc2x0 C.
Thus the role of NO2 in NOx is relatively small or negligible under the carefully operated conditions, or NO2 may be regarded as NO under certain condition which will be explained later.